The mature rat testis contains a soluble form of adenylate cyclase which, unlike the usual membrane-bound form of the enzyme, is not regulated by fluoride ion, guanine nucleotides, metal ions, or hormones. It may represent the catalytic component of the membrane-bound enzyme. The soluble testis enzyme has been purified about 10,000-fold with respect to the whole homogenate of the testis and has a specific activity of approximately 2 micromole cyclic AMP formed per min/mg protein.